Microporous membranes made of polyolefin, such as polyethylene and polypropylene, have been used as separators for batteries such as lithium ion secondary batteries, wherein non-aqueous electrolytic solutions are used. Such a polyolefin separator is known to have high chemical resistance and moreover shutdown function, by which the micropores are blocked up at a temperature close to the melting point of a material due to melting thereof. Therefore, the separator can also play the role of a safety element that, when an abnormal reaction occurs in a battery to raise the battery temperature, terminates the battery reaction by shutdown so as to prevent the battery temperature from rising abnormally. From the viewpoint of the shutdown function, a polyethylene separator having a low melting point is more advantageous. Thus, JP-A-5-25305, JP-A-8-64194 and JP-A-8-29174 disclose techniques where a low density polyethylene having a low melting point is mixed into a high density polyethylene, thereby lowering the shutdown temperature.
On the other hand, when the interior temperature of the battery becomes high, the separator arranged between the electrodes may be ruptured or shrink, so that the electrodes may be contacted with each other, thereby causing an inner electrical short. For the purpose of avoiding such a trouble and keeping the geometry unchanged without rupture of the membrane up to a high temperature, various techniques are disclosed. For example, JP-A-4-126352 and JP-A-5-234578 disclose a blend of polyethylene and polypropylene, and JP-A-7-304110 discloses the lamination of a polyethylene separator and a polypropylene separator.
However, if the heat shrinking force and the heat shrinking percentage in a vertical direction toward the winding direction of a laminate of electrodes and a separator (in other words, in a transverse direction to the separator) are large, a phenomenon occurs that the separator shrinks and goes inside the electrodes when the interior temperature of the battery becomes high. As a result, the electrodes are exposed at both ends of plus and minus electrodes to cause an inner electrical short. Since the battery safety is not ensured for this reason, a further improvement has been desired.
Further, for the purpose of controlling the inner electrical short that may occur when the interior temperature of the battery becomes high, a temperature at which the separator is broken through is important as well as the heat shrinkage properties thereof. The separator becomes susceptible to be broken through by projections, such as electrodes, when exposed to a high temperature, thereby causing the inner electrical short. On the contrary, when the separator is hardly broken through even under a high temperature condition, it is possible to control the inner electrical short and ensure the battery safety. In particular, such a break through temperature of the separator becomes more important for battery safety as the battery capacity becomes higher and the separator becomes thinner.
Accordingly, the property that a temperature at which the pores of the separator are blocked up is low, or the property that the temperature at which the membrane is ruptured is high, alone is hardly able to inhibit the inner electrical short that occurs under a condition where the interior temperature of the battery becomes high and thus to ensure the safety. Hence, it important for the separator that the break through temperature is high and the shrinking force and shrinking percentage in the transverse direction are low under a high temperature condition.
In addition to the above-mentioned safety attributes, properties required for the separator for batteries are high permeability and high strength. For example, JP-A-2-21559 discloses a technique for preparing a separator for batteries by heat-mixing polyethylene having a viscosity average molecular weight of not higher than 300,000 and polyethylene having a viscosity average molecular weight of not lower than 1,000,000 in a good solvent to form a gel film; removing the solvent; and then uniaxially or biaxially stretching the gel film to obtain a separator. However, the disclosure of the specification thereof is actually limited only to a separator obtained through uniaxial stretching, and its porosity (in the present invention, this is defined as void content) is 80% and its strength is pretty low. In addition, the techniques disclosed in JP-A-2-94356 and JP-A-5-21050 are also related to uniaxial stretching in the machine direction.
When a membrane is highly oriented only in a single direction like the membranes obtained through uniaxial stretching as disclosed in those literatures, only membranes which are easily torn, low in piercing strength at ambient temperature and also low in the break through temperature can be obtained.
JP-A-5-318585 discloses a technique, in which a high molecular weight polyethylene having an intrinsic viscosity [η] of 5 or more is used and a membrane is prepared through a biaxial stretching carried out so as to obtain strength in the machine direction higher than that in the transverse direction. The technique comprises carrying out the transverse stretching and the longitudinal stretching in this order, and the magnifying power of the longitudinal stretching is larger than that of the transverse stretching. Therefore, even when the longitudinal stretching is carried out after the extraction of any plasticizer while keeping the length in the transverse direction constant, closure of the pores upon longitudinal stretching cannot be prevented from occurring. As a result, a sufficient permeability can hardly be obtained. Moreover, the piercing strength is low and the break through temperature is low, because of high orientation in the machine direction.
In a technique disclosed in JP-A-11-322989, a microporous membrane, which is low in its transverse shrinkage, and a method for the production thereof are included. However, the disclosure of the specification thereof is limited only to a membrane, which is high in its porosity and low in its piercing strength. Further, the membranes disclosed therein are those obtained through uniaxial stretching, and therefore its break through temperature is low.
JP-A-2003-119306 discloses a microporous membrane, which is low in its transverse shrinkage under high temperature conditions, and a method for the production thereof. The disclosure of the specification thereof is limited only to the lowering of shrinkage, and there is no mention of the strength of the microporous membrane under conditions including high temperature conditions. Further, the examples described therein only illustrate microporous membranes, which are high in gas transmission rate. In such microporous membranes, the low gas transmission rate, the high strength and the high break through temperature, as defined in the present invention, can barely be achieved at the same time.
As mentioned above, a microporous membrane having high permeability and high strength, which is provided with high safety when used as a separator for batteries even under high temperature conditions has not been obtained hitherto.
An object of the present invention is to provide a microporous membrane having high permeability and high strength, which is provided with high safety when used as a separator for a battery even under high temperature conditions.